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334
Radio Broadcast
2-100 TURN HONEYCOMB COILS
.0007 U.F
7"
4 J
FIG. 4
An oscillator for continuous-wave reception
frequencies is being effected, and usually then in an unsatisfactory manner.
A simple and by far the most effective method of procuring audible reception of continuous-wave signals is to set up still another vacuum-tube oscillator (Fig. 4) in the vicinity oj the amplifier circuits, this oscillator being so adjusted as to frequency that a second "beat frequency" — this time an audible one — is produced. In the case which we considered above, where the amplification frequency was 100,000 cycles, the practice would be to set the second oscillator so as to generate a frequency of 100,500 cycles. The "beat frequency" then produced would be an audible one, i.e., 500 cycles, which would give a note having a pitch just slightly lower than that heard when the middle C is struck on the piano. Having set this oscillator to produce the 5OO-cycle tone, it need not be changed, regardless of changes which may be made in the wavelength setting of the receiver. For, it will be remembered, the amplifier has passed to it only currents which have a frequency of 100,009 cycles.
In actual practice, it is found impractical to set the beat note oscillator at 100.500 cycles, because, unless' great pains are taken to shield
the beat-note oscillator carefully, the oscillations produced by it — even though they be very feeble ones — tend to paralyze the action of the amplifier. This problem may be easily solved by setting the oscillator at double the wavelength (half the frequency). There always exists, .in the circuits of the oscillator, harmonics of the true oscillation. If the fundamental oscillation (100,500 cycles) is so strong as to paralyze the action of the amplifier, either the third or fifth harmonic will usually be found of proper strength to give the desired signal without paralysis of the amplifier tubes. The third harmonic is obtained by setting the oscillator at double the wavelength of amplification (6,000 meters or 500,000 cycles, approximately). The fifth harmonic is obtained by setting the oscillator at four times the frequency of amplification (12,000 meters or 250,000 cycles). Figure 3 shows this last arrangement. Attention is called to the fact that the beat note oscillator need be no nearer to the amplifier than about four to six feet.
Care in selecting and placing the tubes available for use in the super-heterodyne is of great importance. That great thing which is to be desired is silence in the amplifier. Noises due to tube faults are a great deterrent. Tubes which show inclination toward noisiness should be used in the latter stages of the amplifier if at all.
I have found many users of multi-stage amplifiers complaining about tube noises which were not tube noises at all. The noises which I have in mind were due to poor connections in the battery circuits. In fact, a poor connection at any point will cause noise, but it seems that the most common cause is the existence of carelessly made connections
LOOP
REGENERATIVE R.F.A.
THESE COILS MAY TAKE FORM OF STANDARD VARIO COUPLER.
FIG. 5
The super-heterodyne circuit with tuned radio-frequency amplification